6Pseudo-Haptic Spring ModelVisual DisplacementForceWe used a Spaceball again. The participants had to press on it (via a self-made set-up) to push the virtual spring displayed on the computer screen. The virtual spring is displayed as similar as possible to the real springs used (=piston of previous slide).We played with the visual displacement of the virtual spring to distort the stiffness perception. When amplifying visually the displacement of the spring on screen, we assumed the user would perceive a ‘soft’ spring. The larger the amplification of the visual displacement, the softer the virtual spring.Thus, we assumed that the stiffness perceived by the participants of our experiments would then correspond to the ratio : (Force applied) / (Visual Displacement); instead of the one : (Force applied) / (Displacement of the finger) –which is constant and equals to the internal stiffness of the spaceball. By using the visual displacement, we could then simulate a large range of PH stiffness.Hooke’s law : F = K.DxForceKv =Displacement of fingerForceKv =Visual Displacement